Effective filtration and drainage systems has proven to be critical in geotechnical and geoenvironmental engineering of fine-grained geomaterials, such as silts or clays, coal combustion products (CCPs) including fly ash and flue gas desulfurization (FGD) materials, and river or sea dredged sediments, etc. For instance, coal combustion products (CCPs) are regularly deposited in landfills which require a leachate collection system beneath the CCPs waste to provide sufficient drainage and reduce the hydraulic head on top of water-barrier liners.
Leachate collection systems have generally consisted of a drainage layer with in-plane lateral flow capacity, and filter layer(s) which retain solid particles in order to maintain soil structural stability while also allowing liquids to pass. Geosynthetic drainage composites, consisting of a drainage geonet core with heat-bonded nonwoven geotextile filter(s), have been widely used for this purpose in many geotechnical drainage applications including municipal solid waste (MSW) landfill leachate collection.
The fabrics used in geotextile filters must therefore have sufficiently large openings to allow the requisite flow of liquid therethrough while at the same time having openings which are sufficiently small to block the solid particles from passing therethrough. Meeting such conflicting requirements can be, and has been, difficult for systems in which a very high content of fine-grained geomaterials is present. Moreover, such fabrics can encounter serious filtration problems such as fines' migration (called “piping”) or filter clogging.
Nonwoven fabrics have commonly been used as geotextile filters. However, tensile strains on such nonwoven fabrics can change the pore opening sizes over the life of the product. Moreover, manufacture of nonwoven fabrics will occasionally result in a fabric which has small light spots with larger openings than other areas of the fabric, and such spots will control the ultimate filtration performance of the material. Woven fabrics have also been used as geotextile filters. However, woven filters typically have only a limited number of the openings per unit area, and fines particles can accumulate at any individual opening and block the water flow (called “surface blinding”). In short, such geotextile filters can encounter difficulty over their useful life, particularly in environments where high contents of fine-grained geomaterials are present.
As a result, it has been common in CCPs landfills to utilize a leachate collection system made of the costly and space-consuming granular materials (e.g., several feet of a fine sand filtration layer plus an aggregate or well-graded bottom ash drainage layer).
The present invention is directed toward providing efficient and inexpensive filtration and drainage even in environments in which high contents of fine-grained geomaterials such as coal combustion product (CCPs) are present.